The above referenced invention relates to an improvement in brake actuator design and specifically improves initial brake system response time.
Hydraulic braking systems have typically been the basis for vehicle braking systems, especially automotive braking systems. Hydraulic systems are used to convert fluid pressure into linear and/or mechanical motion. Such systems allow the source of the hydraulic pressure to be positioned remotely from the cylinders which affect the braking action. These systems comprise an actuator, such as a brake pedal, reservoir fluid which is responsive to pressure applied by the actuator, (such as a master cylinder) and means for converting the hydraulic pressure to a braking force, generally fluid cylinders. Mechanical braking pressure is achieved by utilizing the force of the depression of the brake pedal by the driver to increase the pressure on the master cylinder. Such systems are typically accompanied by a vacuum boost which multiplies the force supplied to the brake pedal, throughout the braking operation. The increased pressure in the master cylinder is then transmitted through fluid lines to the fluid cylinders. The fluid cylinders operate the calipers thereby forcing the calipers and brake pads against the rotors and/or drums which slows the vehicle by frictional force.
Hydraulic systems of the above described type have many disadvantages. These include the large amount of volume and mass that the master cylinder vacuum booster, ABS modulator and hydraulic line add to the completed vehicle. Installation of standard hydraulic braking systems is also complicated and labor intensive. Additionally, the large number of parts and installation also adds to repair and maintenance issues as individual parts reach the end of their useful life. Standard hydraulic braking systems have also become dependent on the vacuum boost to assist in braking operations. However, vehicles such as electric or hybrid vehicles do not produce vacuum as a by-product of the vehicle operation. Thus vacuum boost is not an option on such vehicles.
In order to overcome some of the hydraulic system disadvantages, electric brake systems are known. While there are many variant forms, including electrical hydraulic systems, the use of electric in the variant forms is also referred to as a brake by wire brake system (BBW). BBW describes the ability to activate vehicle wheel brakes via an electric signal generated by an onboard processor/controller as a result of input signals thereto. Brake torque is applied to the wheels without direct mechanical interaction between the vehicle""s brake pedal and the wheel brake.
A particular type of BBW systems is known as a xe2x80x9cdry interface comerxe2x80x9d system (DIC). The typical DIC system operates when a driver inputs a force to the brake pedal. A force sensor and/or travel sensor attached to the pedal transmits an electronic signal to an electronic controller, which in turn sends the signal to the self contained braking device typically located at each wheel of the vehicle. The DIC system is known as a hybrid system in that electric signals are used to generate the type and amount of braking force required at each wheel of the vehicle with electrical wires rather than standard hydraulic brake lines. Located at each comer of the vehicle is a self-contained module which takes the electrical signal and mechanically brakes the vehicle. The self contained module utilizes an individual motor that drives a ball screw piston assembly which pressurizes hydraulic brake fluid to ultimately apply the brake caliper to a rotor at that comer of the vehicle. Such a DIC system significantly reduces assembly cost. The individual modules can be separately assembled and fluid filled prior to the manufacture of the vehicle. DIC modules then only need to be bolted to the automobile during the assembly process and plugged in using standard electrical connections. Finally, the elimination of hydraulic lines stretching throughout the vehicle as well as the elimination of the master cylinder booster, and ABS modulator reduces space requirements within the engine compartment.
Due to the modularity of the DIC system, each of the individual components is preferably kept relatively small while still meeting a baseline brake response. Such a system keeps the DIC module a manageable size and does not overextend the existing electrical system on a vehicle. Such a system works well in most brake system applications. However, in certain instances the vehicle operator desires to decrease the initial brake system response time and shorten the vehicle stopping distance. However, the amount of fluid displacement to achieve pressure is limited by the size and speed of the motor and the gear ratio of the ball screw assembly. While increasing the size of the motor or the piston assembly and/or a combination thereof can decrease the initial brake system response time and result in a shorter vehicle stopping distance, such systems are unnecessary or impractical for most braking applications. Furthermore, such a combination would also result in greater size requirements for the DIC module and could have larger electrical load requirements from the vehicle power system.
Accordingly, it is a feature of the present invention to provide a vehicle braking system which overcomes the disadvantages of prior art vehicle braking systems by temporarily applying a greater amount of fluid to the brake caliper for the same amount of input motor turns. This feature is utilized in those situations in which the vehicle operator has indicated, through the brake pedal, that a shorter vehicle stopping distance is desired. The preferred embodiment of the invention is accomplished with a step piston which is added to the actuator and is preferably activated by closing a normally open solenoid valve. The step piston will result in a higher reaction load on the motor and thus limits the maximum pressure that can be developed while the step piston is in operation. Upon receipt of a signal from the vehicle operator that an improved vehicle stopping distance is desired, the normally open solenoid valve is closed for a very short period of time, after which it is re-opened at a pre-determined pressure during the brake apply, allowing the brake actuator to develop the desired maximum system pressure. The invention allows more initial fluid displacement to the brake caliper, which is not limited by the motor or actuator. The invention improves initial brake system response time and shortens vehicle stopping distance without increasing either the size of the DIC brake module or the electrical system requirements of the vehicle.
This feature, along with other features of the present invention, is achieved in an assembly for providing a braking force to a wheel of the vehicle based upon receipt of an electrical signal. The braking assembly comprises an actuator including a motor that applies and releases a piston within an actuator body. The actuator is fluidly connected to a wheel brake whereby the wheel brake can be applied and released. A valve is interposed between the actuator and the wheel brake on a primary fluid path for opening and closing the fluid connection on the primary fluid path. A secondary fluid path between the actuator and the wheel brake is also provided. A mechanism for increasing the fluid displacement or flow downstream of the secondary fluid path is provided and operates when the valve on the primary fluid path is closed.
In accordance with a more specific feature of the invention, the mechanism for increasing the fluid displacement on the secondary fluid path is preferably a step piston which includes first and second surfaces. The surface area of the second surface is generally greater than the surface area of the first surface and is also located downstream of the first surface. A second valve assembly is also disposed on the secondary fluid path between the actuator and the step piston. The second valve assembly preferably provides an orifice which controls the rate at which fluid flows upstream in the secondary path after the first valve in the primary path has been opened. As such, the gain in fluid pressure initially achieved by use of the step piston at the wheel brake is not lost when the first valve in the primary fluid path is opened.
In accordance with another aspect of the invention, a braking system to provide a braking force to a plurality of wheels of a vehicle is provided. This braking system comprises a plurality of actuators for braking the plurality of wheels, each of the actuators fluidly connected to at least one wheel brake on a primary fluid path. A sensor, for sensing a brake force and/or travel applied to a brake pedal is also provided. The sensed force and/or travel is sent to a controller which electrically generates a brake signal to the plurality of actuators based upon the force and/or travel applied to the brake pedal. A valve is provided on the primary path. This valve is normally opened below a preset value of the rate of change of sensed force or travel applied to the brake pedal. The primary path is closed when the brake signal exceeds that preset value. A secondary fluid path between the actuator and the wheel brake is also provided. The secondary fluid path includes a mechanism for multiplying the fluid displacement in the second fluid path between the mechanism and the wheel brake. A pressure sensor measures the fluid pressure generated in the second fluid path. Once the pressure sensor determines that fluid pressure exceeds a preset value, a signal is sent from the pressure sensor to the controller and thereupon from the controller to the mechanism for opening and closing the primary path, which then reopens the primary path.
In yet another aspect of the invention, a method is provided for initially boosting brake pressure. It includes providing a brake actuator assembly having a fluid displacement rate and providing a wheel brake. The wheel brake actuator assembly is connected to the wheel brake by both a primary fluid path and via a secondary fluid path. The operator of the vehicle generates an electrical brake signal to activate the brake actuator. The electrical brake signal is measured and compared against a preset value. If it is determined that the signal exceeds a preset value the normally open primary path is closed and fluid pressure is diverted to the secondary path. The fluid displacement rate in the secondary path is multiplied and the multiplied fluid displacement is provided to the wheel brake via the secondary path. On determining the fluid pressure in the secondary path exceeds a second preset value, the first primary path is reopened. The method further includes controllably reducing the fluid pressure and restoring displaced fluid in the secondary path after reopening the primary path.
Thus, it is a principal object of the present invention to provide an improved electrical brake assembly which provides a better initial brake system response time.
It is yet another object of the present invention to provide an improved electric brake system which results in shorter vehicle stopping distances.
Still another object of the present invention is to provide an electric brake assembly which provides a better initial brake system response time while not limiting the maximum pressure that can be developed by the wheel brake.
Yet another object of the invention to provide an improved electric brake assembly which does not increase electrical current draw from the vehicle.
Still yet another object of the present invention is to provide an improved vehicle braking system in which fluid displacement can be momentarily multiplied resulting in shorter vehicle stopping distances.
These and other objects of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description of the invention.